What is the highest myopic prescription possible for a IOL?

I wonder if you might be referring to an ICL rather than an IOL?

The difference is that an ICL (Intraocular Collamer Lens) is used for myopia correction by being "added on" over the existing natural lens while an IOL (Intraocular Lens) is used to REPLACE the natural lens, such as in cataract or other lens replacement surgery like PreLex™. IOLs are only minus powers under the most unusual of circumstances.

The most myopic ICL available is about a -16 Diopter power.

_Written by J. Trevor Woodhams, M.D. - Chief of Surgery, Woodhams Eye Clinic